1. Field of the Invention
This invention relates to layered transition metal dichalcogenide storage cells (hereinafter "batteries") with improved capacity and discharge rate or current.
It is an object of the present invention to provide a battery having a high energy storage density.
It is another object of the invention to provide a battery that uses relatively inexpensive material that is easily prepared for the cathode.
2. Description of the Prior Art
In an article in the journal Science, June 1976, M. S. Whittingham reported the development of a new battery storage system based on the intercalation of lithium in titanium disulphide. This work is also described in U.S. Pat. No. 4,009,052 issued Feb. 22, 1977 to M. S. Whittingham and in Belgian Pat. No. 819,672 dated Sept. 9, 1974 (published Mar. 10, 1975).
Earlier work on chalcogenide batteries as evidenced by the paper and patents indicated above covered a large range of possible cathode materials but generally stressed the intercalation of lithium (Li) in titanium disulphide (TiS.sub.2). The resulting Li.sub.x TiS.sub.2 battery system operates at ambient temperatures, has a high energy storage density (about 480 watt-hr/kg), and is highly reversible over the range 0&lt;x&lt;1. When fully charged the cell has a open-circuit emf of 2.5 volts. During discharge, lithium from the electrolyte intercalates in the TiS.sub.2 electrode and is replenished from the Li electrode as the open circuit emf drops to about 1.8 volts. At full discharge the open circuit emf falls to below 1 volt, corresponding to an electrode composition Li.sub.x TiS.sub.2 with x=1. When x=1, full intercalation has been achieved by the deposition of an atomic monolayer of Li between adjacent TiS.sub.2 layers of the original electrode structure.
Several methods are known for accomplishing the intercalation of various materials into layered structures. For example, alkali metals may be intercalated into most transition metal dichalcogenides in liquid ammonia. This technique is described in a paper by W. Rudorff, Chimia 19,489 (1965). Vapor intercalation is described in a paper by Somoano, Hadek and Rembaum, AIP Conference on Superconductivity in d and f band metals, (D. H. Douglas Editor) p. 273 (1972). Another method is described in the paper "Intercalation from Aqueous Solution of Na.sub.2 S.sub.2 O.sub.4 " by Schollhorn, Sick and Lerf, Mat. Res. Bull. 10,1005 (1975). Finally, alkali metals and alkaline earth metals may be intercalated cathodically.
During discharge of a conventional intercalation battery, cations diffuse from the cathode surface (which is in contact with an electrolyte) to the cathode interior. Studies indicate that diffusion occurs along planes in the van der Waals gap (between adjacent transition metal dichalcogenide molecular layers) at a rate governed by the diffusion coefficient, D. It is further understood that diffused cations come to rest at symmetrically recurring sites which arise due to the relative spacing or positioning of adjacent molecular layers. Maximum capacity is apparently reached when all sites are occupied by diffused cations. In a conventional intercalation battery, the rate of diffusion of cations into the cathode is apparently inhibited by the size of the van der Waals gap. Thus, the battery discharge current which is directly proportional to the rate of cation diffusion is also inhibited.